This invention relates to diagnostic methodology for diseases having an arthritic component, including rheumatoid arthritis, osteoarthritis and related conditions.
Rheumatoid arthritis is a widely prevalent, chronic systemic disease thought to have an auto-immune component. See, e.g., Kunkel and Tan, Adv. Immunol. 4, 351-395 (1964). Whether this immune response initiates, perpetuates, or is a consequence of the disease is at present not understood. Both humoral and cellular mechanisms have been proposed heretofore to be involved in the articular and extraarticular manifestations of the disease. Osteoarthritis is an articular disease believed to be distinct from rheumatoid arthritis with bone damage being secondary to cartilage degeneration. This degeneration is thought to be either a primary disease of cartilage or a secondary response to stress-induced microfractures of subchondral bone. For both diseases the differential diagnosis is made currently on clinical history and radiological criteria.
In rheumatoid arthritis, immunoglobulin (Ig) complexes are present and resemble classical antigen-induced immune-complexes in their ability to activate the classical complement pathway, stimulate opsonizing activity of macrophages, evoke immediate-type hypersensitivity upon subcutaneous injection, and the like actions. These complexes consist exclusively of immunoglobulins, which implies that immunoglobulin is both the `antibody` (rheumatoid factor) and the `antigen`. However, these complexes differ from normal immune-complexes in the low affinity of the binding site (Fab region of rheumatoid factors) for the putative determinant, the preference for homologous association (IgG rheumatoid factors) and, in particular, by the fact that the majority of rheumatoid factors (IgM, IgA, IgG) bind to .gamma. class immunoglobulin. It has therefore been argued that there exists in patients with rheumatoid arthritis a structural alteration of .gamma. class immunoglobulins (IgG) which creates an antigenically, and presumably immunogenically active sub-population. See Kunkel and Tan, supra. That this antigenic determinant actually exists, and is localised to the C.gamma.2 domains of altered IgG, has been established by the capacity of rheumatoid factor Fab moieties to bind only to Fc carrying one or both of its C.gamma.2 domains. This and several other lines of evidence have led to the view that the complexes in rheumatoid arthritis patients involve Fab-Fc (IgG) interactions. It has also been postulated that a large number of immune-complexes in rheumatoid arthritis serum consist predominantly, if not exclusively, of self-associated, altered IgG. That is, those IgG molecules which contain anti-IgG activity in their Fab region also carry the structural abnormality on their Fc region. The view however, that this anti-IgG activity is the result of an evoked auto-antibody to an abnormal IgG sub-population, has been challenged by other investigators who presented data which suggested that the self-association of IgG was not the consequence of true antibody-antigen interactions (not an auto-immune phenomenon).
Efforts to investigate the molecular changes in the Fc region giving rise to these phenomena have largely involved the use of rheumatoid factors as the most specific probes available. However, IgG and IgM rheumatoid factors show two puzzling properties. Firstly, they react to a large extent not only with IgG from the same (diseased) individual, but also with IgG from normal individuals and even other species (e.g., rabbit, rhesus monkey). This implies that the Fc binding site on some of the IgG from patients with rheumatoid arthritis also exists on IgG obtained from patients without disease, but presumably in a latent form. Secondly, in some patients anti-IgG activity is associated with a large proportion (25%) of the total IgG pool, and the resulting complexes constitute over 50%. Thirdly, rheumatoid factor - IgG interactions are invariably monovalent, despite the commonly held view that IgG is a structurally symmetrical molecule and should therefore always possess an even number of determinants. Although steric constraints upon the interaction between rheumatoid factor and target IgG might account for this monovalency, it is possible that glycosylation of IgG in fact renders many molecules structurally asymmetrical (though still actually symmetrical with respect to their polypeptides). If the latter were true, the antigenic determinant on Fc could involve oligosaccharide in some way.
There is no evidence in favour of amino acid changes in the Fc region of IgG from arthritic patients. Several reports however have suggested that total serum IgG or IgG-rheumatoid factor from patients with immune-complex diseases (rheumatoid arthritis, systemic lupus erythematosus) is abnormal with respect to its carbohydrate content. See Pope et al., J. Immunol. 115, 365-373 (1975); Hymes et al., J. Biol. Chem. 254, 3148-3151 (1979); and Duc Dodon et al., Immunol. 42, 401-407 (1981).
It is not apparent from these publications how the immune-complex abnormality can be used, if at all, in diagnostic methodology for rheumatoid arthritis. Commonly used procedures for diagnosis of rheumatoid arthritis are based on serologic reactions. According to these tests, the patient's serum is reacted with a variety of serological systems, all of which contain gamma globulin in some form or a component of Cohn Fraction II obtained from plasma or serum, which is largely gamma globulin. If the patient's serum is positive for the disease, the presence of the so-called rheumatoid factor will cause an agglutination or immunoprecipitation reaction which can be compared with control samples. Such reactions can be determined by various well-known definitive tests which include, for example, electrophoresis, Coombs' (antiglobulin) hemagglutination inhibition test, precipitin reaction, gel diffusion, immunoelectrophoresis, nephelometry, radioimmunoassay and flow cytometry.
A typical component in the agglutination assay for rheumatoid factor involves the use of inert carrier particles such as, for example, latex particles. This assay was first described by Singer and Plotz, Amer. J. Med. 21, 888-892 (1956). Modifications of the latex test are described in U.S. Pat. No. 3,088,875 and numerous other patents and publications. Commercial examples of these latex tests for rheumatoid factor are the "Rapi-tex" of Behring Diagnostics, the "RA-TEST" of Hyland Diagnostics and the "RHEUMANOSTICON" of Organon Diagnostics.
Further background information on these conventional diagnostic tests for rheumatoid arthritis can be had by reference to Rose and Friedman, Manual of Clinical Immunology, American Society For Microbiology, Washington, D.C., Second Edition, 1980. See, in particular, Horan and Schenk, "Flow Cytometric Analysis of Serum Autoantibodies Applied to the Detection of Rheumatoid Factor," Chapter 9, pp. 56-59; Agnello, "Method for Detection of Immune Complexes Utilizing Clq or Rheumatoid Factors," Chapter 21, pp. 178-185; and Froelich and Williams, "Tests for Detection of Rheumatoid Factors," Chapter 117, pp. 871-873. See also Schur, "Immune-complex assays: The state of the art," New England Journal of Med. 298 (3), 161-162 (1978).
Despite the substantial amount of scientific effort in the field, it is apparent that no diagnostic test for rheumatoid arthritis is fully reliable. A significant percentage of classical patients with the disease lack the rheumatoid factor. Accordingly, an improved method for the diagnosis of rheumatoid diseases and related conditions would have substantial use.